Accessory battery

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Greg partridge
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Accessory battery

Post by Greg partridge » Thu, 12 Sep 2013, 22:15

Can I have some suggestions on a brand and type of accessory battery for my Daihatsu Mira conversion. What size is needed?

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Johny
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Post by Johny » Thu, 12 Sep 2013, 22:22

Hi Greg. You have to evaluate your own specific requirements.
Unlike ICE cars there is not an absolute need to support 400 A loads for a few seconds (as you know) so small batteries suffice but they may not suit you.
For instance I use a Diamec 9AH VRLA (supported by 40A DC-DC) - but I know that many DIY EVs use the original battery as they like to listen to their stereo for hours on end without the ignition on.

I couldn't justify the weight in mine, but I think a 17 to 20AH AGM/VRLA would be a good compromise.

I guess I'll have to fire up the traction system every half hour or so if we ever take the EV to a drive-in.

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Post by a4x4kiwi » Fri, 13 Sep 2013, 01:20

Yep, about 20Ah is about right.

There is something in the regs about how long lights etc need to be able to work in the absence of an alternator / Dc-Dc
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Post by Tritium_James » Fri, 13 Sep 2013, 17:28

Yep, I pulled a number out of the air and specified that it had to run the hazard lights for 20 minutes. Which is a really small battery, especially if you have LED indicators!

You'll probably want something a bit bigger so your 12V doesn't sag too badly if you have a high-current load that turns on and off, such as the vacuum booster pump for the brakes. Some DC/DCs don't have particularly good response to step changes in the load, so the battery ends up taking the load for a few hundred milliseconds. But if it dips too far it can play havoc with any electronics using the 12V.

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Richo
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Post by Richo » Fri, 13 Sep 2013, 20:47

I hat to laugh the other day when I saw a "load resistor" for people that have problems with their LED indicators!
The resistor didn't seem any cheaper than an aftermarket LED switch unit. Image
Kinda defeats the purpose of using the LED's

I agree 20Ah SLA will be fine.
The 12V LiFePO4 blocks are really too big (40Ah) for an ev.
So the short answer is NO but the long answer is YES.
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Renard
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Post by Renard » Fri, 13 Sep 2013, 23:18

My car has a standard size 17AHr AGM and it works fine -- with a 450W DC/DC converter.
Every so often, Rockby Electronics has them on special for a good price.
Renard

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acmotor
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Post by acmotor » Sat, 14 Sep 2013, 18:07

Nissan and Mitsubishi must have pondered the aux battery question at length. They both ended up with wet lead acid SLI of 30-40Ah size.

EV folk would perhaps have expected at least a lithium battery !!!
However I guess the risk of running flat (ok, fully discharged, nearly 0% SOC) is not lithium territory (without some protection to remove load before low SOC) and not even SLA/VRLA territory. The latter tend to be unsafe and overheat on fast recharge if they have suffered damage in the 0% SOC experience. Wet LA just tends to boil/gas and end up with reduced capacity when abused.
General public jumper lead connection could also have been an issue with a damaged (over discharged) lithium battery that could pull considerable current or suffer dangerous cell failure.

If you are smart with your low SOC management of course, (almost) any battery technology can be used.

Another comment on the Ah size.
Red Suzi used 20Ah SLA.
At 1 C (20A) load it would only give 30 minutes to ~8V (typical LA) and would tend to start at 11V less wiring drops so dull lights.
Given the weight (6kg) and poor load performance it would have been better to use a lithium battery with over discharge protection.

e.g. 20Ah 12V lithium   at true 60Ah continuous discharge it leaves SLA and SLI for dead.
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4Springs
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Post by 4Springs » Sun, 15 Sep 2013, 01:00

Greg partridge wrote: Can I have some suggestions on a brand and type of accessory battery for my Daihatsu Mira conversion. What size is needed?

7AH gel cell lead-acid is just enough for me. Cheap, small, easy to mount securely. And I can swap it with the one in the ride-on lawnmower! I decided that since my DC-DC converter was sized big enough to run everything then I only needed a minimal aux battery.
My traction pack is lead-acid, and so suffers in the cold. On winter nights after a long drive the traction voltage sags quite a bit. In this case the DC-DC converter doesn't keep up as well, and the accessories voltage can sag with it. Enough to notice (dimmed headlights) but not enough to matter.
There was a bit of a discussion here if you are interested:
Controlling the 12v aux battery

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Post by Vectrix150V » Mon, 16 Sep 2013, 16:16

Is there a requirement for an accessories battery in individually constructed vehicles only? If you are only using it to run hazards and brakes I would think a very small battery would suffice.

The Vectrix only runs a DC:DC converter off the main pack, but like others have said, the larger EV's require a secondary battery.

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Post by antiscab » Mon, 16 Sep 2013, 22:30

Vectrix150V wrote: Is there a requirement for an accessories battery in individually constructed vehicles only? If you are only using it to run hazards and brakes I would think a very small battery would suffice.

The Vectrix only runs a DC:DC converter off the main pack, but like others have said, the larger EV's require a secondary battery.


The requirement is across the board, with the exception of motorbikes (even ICE bikes aren't required to have a battery)

motorbikes can be pushed out of the way (usually), not so much for cars

Matt
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2007 vectrix - 156'000km
1998 prius - needs Batt
1999 Prius - needs batt
2000 prius - has 200 x headway 38120 cells

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Post by Vectrix150V » Tue, 17 Sep 2013, 20:38

Ah - makes perfect sense.

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